1. Field of the Invention
This invention relates to improved manufacture of silarylene-siloxane polymers, particularly, silphenylene siloxane polymers, having superior elastomeric properties and improved thermal stability.
2. Description of Related Art
Polymers having alternating silarylene-siloxane units have previously been found to be useful in the aerospace field, as elastomers because of their superior high temperature stability and low temperature flexibility. In recent years, the need for bulkier molecular weight units and higher molecular weight siloxane polymer chains has arisen in order to meet the severe thermal demands for future aerospace missions. However, previous attempts at manufacturing such bulkier siloxane polymers have been unduly limited with regard to the molecular weight achieved, have been undesirably slow, taking numerous hours, if not several days, to achieve completion of the reactions, involve tedious process steps, and require undesirably expensive raw materials.
For example, U.S. Pat. No. 3,325,530 issued Jun. 13, 1967, disclosed early work In the silarylene-siloxane polymer field performed by Wu. Wu's process was limited to a slow, dropwise, addition of diphenyldichlorosilane to a solution of the disilanol [1,4-bis(diphenylhydroxysilyl) benzene], pyridine and tetrahydrofuran. Pyridine served as the hydrogen chloride acceptor, and the reaction mixture was stirred, taking for example sixteen hours to complete the reaction. Then a resinous product was separated, having the formula: ##STR2## where n is 3 to 50 However, this process is limited to providing polymers having the number of repeating units ranging from not much more than 3 to 50.
It was later disclosed by the General Electric Company that the limitation regarding the above-described polymerization range of between 3 and 50 was because of polymer backbone cleavage from an equilibrium reaction between (a) the carbon-silicon bonds in the main polymer chain, and (b) the hydrogen chloride by-product, which reaction purportedly takes place before by-product neutralization by the pyridine acid acceptor. This work is described in C. Eaborn, Organosilicon Chemistry, Second International Symposium on Organosilicon Chemistry, Bordeaux, Jul. 9-12, 1968, Butterworth, London, Volume 11, page 375.; and V. Bazant, V. Chalovsky, and J. Rathousky, Organosilicon Compounds, volume 1, pages 225-226, Academic New York (1965).
Silarylene-siloxane elastomeric polymers for modern day and future use in the aerospace industry require much higher degrees of polymerization ranging, for example, from 90 to about 350 and molecular weights ranging from 27,000 to about 123,000. Accordingly, the Wu process has long been abandoned while the industry, for the most part, has turned to "Pike's reaction" for commercial production of the higher molecular weight polymers. R. M. PIke, Journal of Polymer Science, volume 50, page 151 (1961). Pike reported that diaminosilanes condense with silanols to form polysiloxanes under extremely mild conditions in which "basic rather than acidic" by-products were formed without self-condensation of silanols. This route has been commercially adopted, as an alternative to the reaction of dlchlorosilanes with disilanols because of the apparent elimination of the above described polymer backbone cleavage from the attack of reaction by-products. See R. L. Merker and M. J. Scott, Journal of Polymer Science, A, 15 (1964); L. W. Breed, et. al. Journal of Polymer Science, A-1, 5, 2745 (1967); R. E. Burks, Jr., et. al., Journal of Polymer Science, Polymer Chemistry Edition, 11, 319-326 (1973); C. U. Pittman, Jr., et. al., Journal of Polymer Science, Polymer Chemistry Edition, 14, 1715-1734 (1976), all of which report various "Pike reactions" of diaminosilanes having the formula: ##STR3## with disilanols having the formula: ##STR4## to produce the higher molecular weight silphenylene siloxane polymers of higher degrees of polymerization having the formula: ##STR5## where R.sub.1 -R.sub.6, are all either methyl or phenyl groups and n is an integer above 90. However, these reactions require the slow addition of the diaminosilanes, which are less readily available than dichlorosilanes, to a refluxing solution of the disilanols in toluene. These procedures also involve tedious incremental work and unduly long reaction times, where precipitation and purification sometimes take as long as 24 hours.
More recently, Yu-Chin-Lai, et. al., Journal of Polymer Science, Polymer Chemistry Edition, 20, 2277-2288 (1982), reinvestigated the preparation of alternating silphenylene-siloxane polymers from reacting disilanols and dichlorosilanes, partly because these monomers are more readily available than diaminosilanes. However, this required particular monitoring techniques in order to achieve the more desirable higher molecular weight polymers from dichlorosilane monomers. The process also flushed inert gas through the reaction medium to rid the system of HCI in an effort to avoid the polymer cleavage problem. Although Yu-Chin-Lai determined that the degree of polymerization could be substantially enhanced, the need to follow this particularly meticulous and extensive monitoring technique together with the need to continually flush the reaction medium with nitrogen while the reaction is allowed to reach equilibrium, is commercially undesirable.
Accordingly, it would be a substantial advancement, and an unexpected discovery in the art, to manufacture higher molecular weight silphenylene-siloxane polymers having over 50 units, using the dichlorosilane monomer, in a quick and instantaneous reaction which negates the need for flushing inert gas through the reaction medium, negates having to slowly add the monomers, and negates continually monitoring the development of polymerization.